A typical DC-DC converter is depicted in FIG. 1. When it is turned on, the output voltage VOUT on the hold capacitor Cout is practically null. The generator of PWM driving signals (PWM GENERATOR) of the half-bridge stage keeps the high-side transistor of the half-bridge saturated until an error signal VERROR, that is essentially a low-pass filtered replica of the difference between a reference voltage VREF and a scaled replica of the output voltage VOUT, decreases.
A tank capacitor Cout is charged through an inductance Lout, that is initially discharged, thus an uncontrolled overshoot of the current through the inductor Lout and a consequent uncontrolled overshoot of the output voltage VOUT with respect to the pre-established regulation value, determined by the voltage VREF, is generated.
The first effect may cause a breach of the specifications of maximum value of the current that can be absorbed from the line VIN or of the power consumption in the high-side of the half-bridge. The second effect may damage the circuits supplied by the regulator.
In order to address this, DC-DC regulators may include a so-called “soft-start” circuit, illustrated in FIG. 1, that drives the voltage VERROR with a pre-established voltage ramp. An exemplary embodiment of the soft-start circuit of FIG. 1 is depicted in FIG. 2. A voltage ramp generated by the generator RAMP GENERATOR is supplied to a clamp operational amplifier that makes the error voltage VERROR increase gradually. The duty-cycle of the PWM driving signal of the high-side transistor is gradually increased and thus overshoots of the current through the inductor Lout and of the voltage on the capacitor Cout may be limited.
The slope of the ramp is established as a function of the characteristics of the regulator and of the desired maximum current during the soft-start phase. Typically, the voltage ramp is generated by injecting a constant current through the tank capacitor. The slope is generally of about 1V/ms and this may be hard to obtain with a fully integrated architecture.
For example, U.S. Pat. No. 5,917,313 discloses a stepwise voltage ramp generator based on a switched capacitor circuit. U.S. Pat. No. 6,316,926 discloses a digital soft-start circuit for a DC-DC converter.
Other voltage ramp generators may, however, be desirable.